Cleaning or surface preparation of silicon wafers with or without various layers of films is very critical in integrated circuit manufacturing processes. The removal of particles and contaminants from wafer surfaces is performed at several critical process steps during the fabrication of integrated circuits. At a 0.18 μm technology node, 80 out of 400 steps or 20% of the fabrication sequence is dedicated to cleaning. The challenges of cleaning technology are multiplied by the varied types of films, topographies, and contaminants to be removed in front-end-of-line (FEOL) and back-end-of-line (BEOL) cleaning processes. Removal of particles is an important part of this cleaning.
For the defect-free manufacture of integrated circuits, the International Technology Roadmap for Semiconductors (ITRS) indicates that the critical particle size is half of a DRAM ½ pitch [1]. Thus, at the 130 nm technology node, the DRAM ½ pitch being 130 nm, the critical particle size is 65 nm. Therefore, particles larger than 65 nm size must be removed to ensure a defect-free device.
Such small particles are difficult to remove since the ratio of the force of adhesion to removal increases for smaller-sized particles. For submicron particles, the primary force of adhesion of the particles to a surface is the Van der Waals force. This force depends on the size of the particle, the distance of the particle to the substrate surface, and the Hamaker constant. The Van der Waals force for a spherical particulate on a flat substrate is given as in equation 1:
                              F          ad                =                                            A              132                        ⁢                          d              p                                            12            ⁢                          Z              0              2                                                          (        1        )            where A132 is the Hamaker constant of the system composed of the particle, the surface and the intervening medium; dp is the particle diameter; and Z0 is the distance of the particle from the surface. The Hamaker constant A132 for the composite system is given as in equation (2):A132=A12+A33−A13−A23  (2)
The relationship of the Hamaker constant of two dissimilar materials is expressed as the geometric mean of the individual Hamaker constants as Aij=(Aii*Ajj)1/2 where Aii and Ajj are the Hamaker constants of materials i and j. It is calculated theoretically using either the Lifshitz or the London models. The Hamaker constant for particles and surfaces used in integrated circuit manufacturing processes is given in literature [2, 3] and is less when the intervening medium is liquid as compared to air. The Van der Waals force, being directly proportional to the Hamaker constant, is therefore reduced when there is a liquid layer between the particle and the surface.
In addition to the difficulty in removing small particles from the surface, there are various types of organic and metal-organic contaminants which must be cleaned away. As an example, etching is done in integrated circuit device fabrication processes at a number of steps both in FEOL and BEOL to form patterns. The etch is often performed by reactive ion etching (RIE) which generally has a physical and a chemical component to it. Following this process, the etch residues, which are polymeric sometimes with metallic contaminants embedded inside the polymeric matrix, have to be removed. The photoresist film left behind after the etching also has to be removed prior to the next step in the integrated device fabrication process. In case of chemical-mechanical polishing, the polishing steps may use Cerria, alumina or silica slurries. After polishing, the slurry and any residues from the slurry additives need to be cleaned from the wafer surface before the next layer of film is deposited. Thus, there is a wide variety of residues, particles and other foreign materials which need to be cleaned both from the surface of the wafer as well as inside any etched features.
The prior art processes use CO2 or argon cryogenic sprays for removing foreign materials from surfaces. As examples, see U.S. Pat. No. 5,931,721 entitled Aerosol Surface Processing; U.S. Pat. No. 6,036,581 entitled Substrate Cleaning Method and Apparatus; U.S. Pat. No. 5,853,962 entitled Photoresist and Redeposition Removal Using Carbon Dioxide Jet Spray; U.S. Pat. No. 6,203,406 entitled Aerosol Surface Processing; and U.S. Pat. No. 5,775,127 entitled High Dispersion Carbon Dioxide Snow Apparatus. In all of the above prior art patents, the foreign material is removed by physical force involving momentum transfer to the contaminants where the intervening medium between particle and substrate surface is air. Since the force of adhesion between the contaminant particles and the substrate is strong, the prior art processes are ineffective for removing small, <0.3 μm, particles.
U.S. Pat. No. 6,332,470, entitled Aerosol Substrate Cleaner, discloses the use of vapor only or vapor in conjunction with high pressure liquid droplets for cleaning semiconductor substrate. Unfortunately, the liquid impact does not have sufficient momentum transfer capability as solid CO2 and will therefore not be as effective in removing the smaller-sized particles. U.S. Pat. No. 5,908,510, entitled Residue Removal by Supercritical Fluids, discloses the use of cryogenic aerosol in conjunction with supercritical fluid or liquid CO2. Since CO2 is a non-polar molecule, the solvation capability of polar foreign material is significantly reduced. Also, since the liquid or supercritical CO2 formation requires high pressure (greater than 75 psi for liquid and 1080 psi for supercritical), the equipment is expensive.
As such, there remains a need for a more efficient and effective removal process of contaminants, including particles, foreign materials, and chemical residues, from the surfaces of substrates such as semiconductor wafers, metal films, dielectric films, and other substrates requiring precision cleaning.